1. Field of the Invention
The present invention deals with a lighting unit with low and high light levels, energized from a conventional ac source, and in which the principal source of light is an arc lamp supplemented by a standby filamentary lamp providing light during starting of the arc lamp and during low light levels.
2. Description of the Prior Art
The present invention deals with a lighting unit in which the principal source of light is a high pressure discharge lamp having up to six times the efficiency of an incandescent lamp. High pressure metal vapor lamps have been available for some time in high power units. Recently, as disclosed in U.S. Pat. No. 4,161,672, smaller low wattage metal halide lamps with efficiencies approaching those of the larger size have been invented. Such lamps are an energy efficient replacement for the incandescent lamp.
The power supply of the present lighting unit employs a high frequency power supply in which a ferrite transformer controlled for non-saturated operation, a transistor switch, and a trigger oscillator are the principal components. The inverter and ferrite transformer herein described are the subject of U.S. Pat. Nos. 4,350,930 and 4,307,334.
In the prior application Ser. No. 47,972 the high and low settings of the lighting unit are achieved by inserting one or two resistances in series with the arc lamp. The present invention is directed to an alternative solution to the dimming problem in which the low light level is provided by an incandescent element and the high light level by the arc lamp. It is of course desirable that the lighting unit be usable in a conventional as well as a three-way socket.